A contractor says he has been forced to chopper foreign workers into a construction site, south-west of Melbourne, because of a blockade by unemployed workers.

The men have been blockading the Werribee Water Treatment plant for a week, to protest against the use of four Filipino workers on 457 visas.

Briagolong Engineering says the foreign workers provide specialist tank building skills, which cannot be found locally.

But the Leigh Diehm, of the Australian Manufacturing Workers Union (AMWU) says that is not true.

"That is an absolute outrage for Briagolong to suggest they have to import that labour," he said.

"I know for a fact that they employ people in Papua New Guinea from Australia who were more than skilled to do that work, exactly the same work. And some of those people that they did employ are unemployed now and would love to work out there."